COMBINED GAS AND HEAT EXCHANGE IN EXTRACORPOREAL CIRCULATION

COMBINED GAS AND HEAT EXCHANGE IN EXTRACORPOREAL CIRCULATION

COMBINED GAS A N D HEAT EXCHANGE I N EXTRACORPOREAL CIRCULATION Nicholas P. D. Smyth, M.D. (by invitation), Washington, and Brian B. Blades, M.D., ...

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COMBINED GAS A N D HEAT EXCHANGE I N EXTRACORPOREAL CIRCULATION Nicholas P. D. Smyth, M.D. (by invitation), Washington,

and Brian B. Blades,

M.D.,

D. C.

I

N extracorporeal circulation for cardiac surgery, the functions of both gas and heat exchange require dispersion of blood over a large surface area. There are many advantages in combining both in one unit. Some are obvious, such as reduction in priming volume, length of tubing, and number of connec­ tions used. Others, less obvious, might include reduced trauma to the blood and reduced danger of gas embolism as a result of warming the blood. We selected the vertical-screen oxygenator for modification, because of its efficiency,1'2 and because the absence of moving parts would make the task theoretically easier. Since beginning this work, we have become aware of the successful application of this principle of combined gas and heat exchange to the bubble oxygenator 3 ' * and the disc oxygenator. 5 - 6 ' 7 The proposed modification consists in replacing the screens in the oxy­ genator by thin metal plates, through the center of which the heat exchange fluid circulates, and on the surface of which the blood is filmed for gas and heat exchange. We have previously reported preliminary results with a single test plate in which the filming surface was etched in a pattern resembling that of the standard wire screen.8 In the present study, the heat exchanger capacity of this plate and one with a smooth filming surface are compared, and the oxygenating capacity of each is contrasted with that of the standard wire screen. METHODS

The two test plates were each made from two pieces of No. 304 stainless steel measuring 12 inches in height by 16 inches in width by 6%,ooo of an inch in thickness. On one side of each piece of steel, mirror image halves of the cen­ tral duct system were chemically milled to a depth of 1%,ooo °f an inch (Fig. 1). The paired plates were then fastened together by means of silver solder in one plate and cement* in the other in order to form a single plate % of an Prom the Department of Surgery, The George Washington University School of Medicine, Washington, D. C. Supported in part by U. S. Public Health Service Grant No. H5853. Read at the Forty-third Annual Meeting of The American Association for Thoracic Sur­ gery at Houston, Texas, April 8-10, 1963. ♦Scotch-Weld No. 588, Minnesota Mining and Manufacturing Company, St. Paul, Minnesota, 629

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inch in thickness containing a central duct system with a cross-sectional area of 3 %,ooo of an inch by % of an inch. The edge of each plate was then sealed with a continuous Heliarc welded seam. On each side of both plates a pattern was chemically milled to form a " w e i r , " ^ , 0 0 0 of an inch deep and % of an inch high across the top of the plate. In one plate a 3 mm. by 1 mm. diamond relief pattern, %,0oo of an inch deep, was chemically milled on the remainder of its surface. The filming surface thus resembled the standard Tyler No. 538 type 304 stainless steel Ton-Cap screen,1' 2>9 although the depth of the pattern was considerably less than the thickness of the screen. The filming surface of the second plate was smooth (Pig. 2). Inlet and outlet manifolds were welded to the bottom of each plate, with the use of standard hose threads for connection to the heat exchange fluid system (Fig. 3). Both plates were finally electropolished.

Fiff. 1.—View of plate sections which shows heat exchange duct pattern. The fluid enters the vertical duct and is carried first to the top of the plate.

The finished plates were each the same size as the Kay-Gaertner screen,2 and, during testing, each was housed in the Lucite box of a Kay-Gaertner screen oxygenator,* modified to take the single plate, t Temperature change was ef­ fected by a Therm-O-Rite unit.i Blood and heat exchange fluid inflow and out­ flow temperatures were recorded electrically. § Oxygenation was studied on each of the test plates and a standard wire screen by partial bypass from a dog under barbiturate anesthesia. The venous blood, obtained from the external jugular vein, was circulated across the test plate or screen and returned to the femoral artery. Humidified oxygen was passed through the oxygenator casing at a rate of 2 to 3 liters per minute. Blood flow rates, varying from 100 c.c. per minute to 330 c.c. per minute were used •The Mark Company, Randolph, Massachusetts. •(International Medical Instrument Corp., Stoneham, Massachusetts. tTherm-O-Rite Products Corp., Buffalo, New York. lYellow Springs Instrument Co., Yellow Springs, Ohio.

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Fig. 2.—A, Plate with diamond pattern Aiming surface. Inset shows detail of diamond pattern. B, Smooth plate. Note difference in "weir" pattern. Most of the vertical "spacer bars" have been removed to minimize obstruction to blood flow.

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in sixty-one studies on 18 dogs. Each flow rate was established during recirculation with the animal excluded from the circuit. Bypass was then started at that flow rate and maintained for 5 to 15 minutes to clear all oxygenated recirculated blood from the system. Blood samples were then drawn simul­ taneously from the venous and arterial lines of the oxygenator. Recirculation was started again and the process was repeated at a different flow rate (Fig. 4). Oxygen determinations were made according to the method of Van Slyke and Neill.10 All values obtained were corrected to standard temperature and pres­ sure, a standard hemoglobin value of 14 Gm., and p H 7.4.9'1X

Fig. 3.—Composite photograph of one test plate which shows relationship of central duct system to filming surface and manifolds. Inflow manifold is on left.

Heat exchange was studied by circulating 1,000 c.c. of blood from heparinized dogs across each of the plates in a closed system. In twenty studies with each plate, blood at 22° C. to 36 °C. was filmed on each plate at flow rates of 100 to 600 c.c. per minute, and the heat exchange fluid, at 0° C. to 4° C , was circulated through the plate at a rate of 2,800 c.c. per minute and a pressure of 4 to 14 pounds per square inch. Higher flows and pressures were not used because of the risk of leakage in the plate (Pig. 5). Temperature recordings were made at one minute intervals until the blood leaving the plate reached a temperature of 2° C. to 10° C. The heat exchange fluid system was then emptied and the cold alcohol-water mixture was immedi­ ately replaced by water at 42° C. This was circulated through the plate and

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temperature recordings were again made every minute until the blood leaving the plate reached a temperature of 36° C. to 38° C. Heat exchange efficiency was calculated for each series of readings by expressing the change in temperature of the blood as a percentage of the dif­ ference between the inlet blood temperature and the inlet water temperature. 5 The average value for each cooling and heating cycle was then plotted against the blood flow rate.

FROM ANIMAL (VENOUS)

Fig. 4.—Diagram of oxygenator test circuit.

OUTFLOW) (-HEAT EXCHANGE INFLOW ) FLUID

TEMPERATURE RECORDER

Fig. 5.—Diagram of heat exchanger test circuit.

RESULTS

Satisfactory oxygenating capacity has been denned as the ability of an oxygenator to raise the saturation of blood from 65 to 95 per cent. 1 ' 9 In our preparation, the saturation of the venous blood of the anesthetized dog varied widely in different animals, and at different times in the same animal, which made interpretation of the results difficult. Following the suggestion of Theye, Donald, and Jones, 9 we corrected all values to a standard venous saturation of 65 per cent, thus rendering the values obtained for arterial saturation com-

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BLADES

parable. The results are shown in Pig. 6. There is still a considerable scatter in the arterial saturation data. This was noted also by Theye, Donald, and Jones 9 and attributed by them to unreproducible variations in film uniformity. The data suggest that the maximum flow at which satisfactory oxygenation is consistently obtained is 200 c.c. per minute for the plates compared with 260 c.c. per minute for the screen. The results are comparable to those of Theye, Kirklin, and Fowler 12 who found corresponding maximum flow rates of 210 c.c. and 275 c.c. per minute using larger plates and screens measuring 12 by 24 inches. Our data, surprisingly, did not show any superiority of the diamond pat­ tern plate over the smooth plate, but did confirm the expected superior oxy­ genating capacity of the screen. 1 ' 12 However, we were not able to obtain satis­ factory oxygenation at the maximum flow of 300 c.c. per minute originally claimed for the Kay-Gaertner screen.2

• •



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'

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DIAMOND

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SMOOTH



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Fig. 6.—Saturation of arterial blood obtained at various flow rates for each of the test plates and a standard wire screen. All values have been corrected to a venous standard of 65 per cent saturation. The maximum flow rates at which consistently satisfactory oxygenation was obtained on the plates is indicated by the cross-hatched bar on the left. The bar on the right indicates the equivalent flow rate for the screen.

The heat exchange efficiency of the plates is shown in Fig. 7. The difference between cooling and heating efficiency was due to the smaller thermal gradient generally available during rewarming. 5 The drop in heat exchange efficiency instead of the expected rise at the lowest flow rate (Fig. 7, A) was thought to be due to instability of the blood film at low flows caused by warping of the plate during welding, 8 and to an inadequate " w e i r " pattern producing ex­ cessive obstruction at low flows (Pig. 2, A). These deficiencies were corrected in the second (smooth) plate by the use of cement to fasten the plate together and a change in " w e i r " design (Fig. 2, B), and the expected improvement in efficiency was demonstrated in the cooling cycle. However, the drop in efficiency

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DIAMOND

PATTERN

635

PLATE

9080

O u. 3 0 -

0

100

200

300

400

900

600

BLOOD FLOW RATE IN CC./MIN.

Pig. 1A.—Heat exchange efficiency curve of diamond pattern plate. At lowest blood flow rate, true values ( x ) are lower than expected values (solid line).

SMOOTH PLATE

Z UJ

70

.7-

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<3 .3 z

40

? 4

30

.3

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BLOOD FLOW RATE IN CC./MIN.

Pig. IB.—Heat exchange efficiency curve of smooth plate. At lowest blood flow, cooling efficiency is now maximal, but warming efficiency (X) is less than expected value.

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in the rewarming cycle persisted (Fig. 7, B). This was found to be due to a different phenomenon. When water at 42° C. was circulated through the plate on which blood at 4° C. was filmed there was always marked thinning of the film. At the lower flow rates this progressed on occasion to complete breakdown of the film on portions of the plate, thus reducing the blood film area available for heat exchange (warming). At the optimal blood flow rate for oxygenation of 200 c.c. per minute, a heat exchange efficiency of 68 per cent was obtained in the cooling cycle and 58 per cent in the warming cycle for the diamond pattern plate. Equivalent values for the smooth plate were 64 and 58 per cent, respectively. DISCUSSION

A blood flow rate of 200 c.c. per minute appears to be the maximum at which satisfactory oxygenation consistently occurs, although both plates on occasion performed satisfactorily up to a maximum of 220 c.c. per minute (Fig. 6). The use of a deeper etch on the diamond pattern might produce greater turbulence and, hence, better oxygenation, 1 perhaps approaching the capacity of the screen. Such a gain, if attainable, would have to be balanced against the technical difficulty of deep etching of fine patterns in stainless steel, and the greater difficulty of polishing and cleaning such a surface. Since the complete unit will consist of a number of identical plates, its capacity is predictable, and will depend on the number of plates used. A small loss in efficiency per plate can be easily compensated by an increase in the number of plates used.12 With an oxygenating capacity of up to 200 c.c. per minute, a twenty plate unit should provide satisfactory oxygenation for flow rates up to 4,000 c.c. per minute which is an adequate maximum perfusion rate. The diamond pattern plate showed slightly greater heat exchange efficiency than the smooth plate, probably due to the reduced mass of metal between the heat exchange fluid and the blood as a result of the etched pattern. The most significant blood flow rate for heat exchange is, of course, the optimal flow rate for oxygenation, namely, 200 c.c. per minute. The heat ex­ change efficiency should be the same for twenty plates as for one. At a blood flow rate of 200 c.c. per plate per minute, or 4,000 c.c. per twenty plate unit per minute, the cooling efficiency would be 64 to 68 per cent. The cooling ef­ ficiency at this flow rate of a number of heat exchangers in general use is com­ pared in Table I. The comparison shows that the multiplate heat exchanger should be superior to any other on which we have comparable data, 5 ' 13 " 15 with the possible exception of the large Olson unit. 13 The efficiency of this heat exchanger is due mainly to its very large surface area of 0.27 square meters per plate or 5.4 square meters per twenty plate unit. The latter figure is estimated to be from two to fifty times greater than the surface area of the heat exchangers listed in Table I. 8 ' 13 " 17 Another significant factor is that the blood is filmed on the only free surface of the heat exchanger. There is no outer casing, as in all separate heat exchangers, from which heat is lost to, or gained from, the atmosphere.

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November, 1963 TABLE I. H E A T EXCHANGE (COOLING) EFFICIENCY AT BLOOD F L O W KATE OF 4,000 C.C./MIN. TYPE OF H E A T E X C H A N G E E

Heat exchange plate oxygenator* Diamond pattern Smooth Olson" Hufnagel (Brunswick) i* Esmond et al. 1 5 Urschell et al. 5 Brown-Harrison" Lawrence (Saras) 1 T •Based on single plate efficiency. See text.

|

EFFICIENCY (PERCENTAGE)

68 64 66 57 40 36 32 30

The disadvantages of the design include the high resistance of the duct system which allows a very small heat exchange fluid flow rate, the tendency of the film to break during rewarming at low flows—obviously detrimental to oxygenation—and the considerable weight of the unit. The duct system is being re-designed to reduce resistance and increase the heat exchange fluid flow rate. This should allow the efficient use of a smaller thermal gradient during rewarming and eliminate the tendency of the film to break. To reduce weight we are studying the possibility of using aluminum plates covered with Teflon or Mylar. The more efficient duct system it is hoped will offset the insulating effect of the plastic. Unless a deeply etched diamond pattern can be shown to contribute sig­ nificantly to the stability of the blood film during rewarming, the simpler smooth plate would seem to be preferable. SUMMARY

The advantages of combining oxygenation and heat exchange in a single unit are described. A modification of the vertical-screen oxygenator is proposed in which the screens are replaced by plates, on the surface of which blood is filmed for oxy­ genation, and through the center of which liquid flows to effect heat exchange. The oxygenating capacity of a smooth plate, a wire screen, and a plate with a surface pattern etched to resemble the screen are studied and compared. The heat exchange capacity of the plates are studied and compared with separate heat exchangers. The data suggest that a multiplate unit should provide adequate oxygenat­ ing capacity and superior heat exchange capacity. Design changes are suggested to reduce weight, improve rate of flow of heat exchange fluid, and eliminate instability of blood film during application of maximal thermal gradient in rewarming. We acknowledge with thanks the technical assistance of Mr. William F . Barton, M.E., in the design and fabrication of the heat exchange-oxygcnator plates. REFERENCES 1. Miller, B. J., Gibbon, J . H., and Gibbon, M. H . : Eecent Advances in the Development of a Mechanical Heart and Lung Apparatus, Ann. Surg. 134: 694-708, 1951.

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2. K a y , J . H., and Gaertner, R. A.: A Simplified P u m p Oxygenator W i t h Flow Equal to Normal Cardiac Output, S. Forum 7 : 267, 1957. 3. Gollan, F . : Discussion on Physiology of Perfusion in Extracorporeal Circulation, Spring­ field, 111., 1958, Charles C Thomas, Publisher, p . 212. 4. Zuhdi, N., Kimmell, G., Montroy, J . , Carey, J . , and Greer, A . : Apparatus for Hypothermic Perfusion—Clinical Application, Am. Surgeon 26: 446-450, 1960. 5. Urschel, H. C , Greenberg, J . J., and Both, E . J . : Rapid Extracorporeal Hypothermia, N a v a l Med. Research I n s t . Report, Aug. 6, 1959. 6. Osborn, J . J., Bramson, M. L., and Gerbode, F . : A Rotating Disc Blood Oxygenator and I n t e g r a l H e a t Exchanger of Improved Inherent Efficiency, J . THORACIC & CARDIOVAS. SURO. 39: 427-437, 1960.

7. Gebauer, P . W., Brainard, S. C , Mason, C. B., and Connor, M.: A Temperature Control Unit for a Commercial Disc Oxygenator, Hawaii M. J . 19: 651-653, 1960. 8. Smyth, N . P . D., Blades, B., and Barton, W. F . : Combined Gas and H e a t Exchange in Extracorporeal Circulation, Proc. Soc. Exper. Biol. & Med. 112: 803-811, 1963. 9. Theye, B . A., Donald, D. E., and Jones, R. E . : The Effect of Geometry and Filming Surface on t h e Priming "Volume of t h e Vertical-Film Oxygenator, J . THORACIC & CARDIOVAS. SURG. 4 3 : 473-480, 1962.

10. V a n Slyke, D. D., and Neill, J . M.: The Determination of Gases in Blood and Other Solutions by Vacuum Extraction and Manometric Measurement, J . Biol. Chem. 61: 523-573, 1924. 11. Handbook of Respiration, National Academy of Sciences, National Research Council, Philadelphia, 1958, W. B . Saunders Company. 12. Theye, R. A., Kirklin, J . W., and Fowler, W. S.: Performance and Film Volume of Sheet

and

Screen

Vertical-Film

Oxvgenators,

J.

THORACIC & CARDIOVAS. SURG.

43: 481-488, 1962. 13. Technical Brochure; Edward A. Olson Co., Inc., Ashland, Mass.: Olson Blood Heat Exchanger. 14. Technical Brochure; Brunswick Manufacturing Co., Inc., Boston, Mass.: Brunswick Blood H e a t Exchange Instrument. 15. Esmond, W. G., A t t a r , 8., Stram, J., Demetriades, A. D., .Turf, A., Gold, M. I., and Cowley, R. A . : Profound Hypothermia With Simplified Equipment: A Disposable Stainless Steel H e a t Exchanger of High Efficiency, J . THORACIC & CARDIOVAS. SURG. 42: 563-574, 1961.

16. Technical Brochure; Ward Laboratories, Durham, N . C : Brown-Harrison Blood Heat Exchanger Assembly. 17. Technical Brochure; Sarns Inc., Ann Arbor, Mich.: Lawrence Blood Heat ExchangerBubble T r a p Unit. (For Discussion, see page 649)